Reviving a "sleeping" CRT - UK Vintage Radio Repair and Restoration Discussion Forum

Zelandeth · 13th May 2007, 12:04 am

We all know the situation. You've got a CRT which has been sitting unused for 20, 30 years...and it's showing up as low emission.

We all know that it's worth running the tube for a good length of time (24, 48 hours?) before condeming it though, or risking "boosting" it to see if anything happens.

I know that two things happen when it's left running:

1. The emissive coating on the cathode is drawn back to the surface to allow emission to take place properly again.

2. The getter is reactivated by the heat from the heater, which helps to clean up any air (within reason) which may have found its way into the tube in the years since it was last used.

...however, you've got a set which isn't really in a condition to be run for any length of time for risk of self-destructing. Either that, or you don't want something using 200 odd watts of power left running for a day! (Don't know about anyone else...but I know that my old KB does a good impression of a space heater too)

This is my question:

For this to work, is it just a matter of the CRT heater running, or is the process reliant on the full EHT being present as well?

i.e. is it possible to improve things just by connecting the CRT up to a suitable heater supply and leaving it running? Or would one need to actually have it running in the set?

I tend to think it's unlikely to be that simple...otherwise everyone with this question would have just found a suitable heater transformer and left it running!

Thoughts?

Zelandeth · 13th May 2007, 1:22 am

I know there are ways to revive old, low emission CRTs, at least for a while.

What I'm talking about here though is a tube which is quite possibly in good health (but you don't know that for certain), but is really just needing a good old run. Obviously, given that the boosting proceedure has a certain risk attached to it, you'd not want to do that unless you knew the tube was low emission. In which case there's little to lose.

In this case, the question is more whether it's actually worthwhile sticking a 6.3V supply onto the heater pins in the CRT (or whatever the correct voltage is for the tube in question), and leaving it to glow away to itself for a day or so. Or without the EHT present, is that pointless?

Danny · 13th May 2007, 3:43 am

You need to draw some current through the tube as well as the heater .
This is done when a tube is new, so some HT is used at a controlled way and the tube used as a diode, but limit the current to microamps.
EHT isn't needed to age a tube!

Heatercathodeshort · 13th May 2007, 8:10 am

Hello,
As Danny says, a very small current needs to flow between the cathode and grid to liven up a very tired CRT. This is carried out when the tube is built and is called 'ageing'. As long as there is a current, no matter how small passing between the cathode and grid, the relivening process will slowly take place. It might be worth experimenting with a common tube of any type, a resistor network supplying a very small positive voltage to the grid and a microamp meter connevted in the cathode. [Getters used with vintage tubes were generally large and cope well with any microscopic gas molecules. The later tubes such as the A50-120WR had much smaller assemblies and a visual inspection of the getter area will show a lack of the familiar silvery appearance.]
Start with just a few MICROamps and then increase it to say 10 for a few hours. This will not harm the tube in any way. If this fails to produce a result the tube reactivator circuit linked by Steve should do the trick without ruining the tube.
Regards, John.

Heatercathodeshort · 14th May 2007, 7:26 pm

Hello Paul,
The voltage needs to be around 30-90v to stimulate the initial 'kick' through a fixed resistor or number of resistors in series. These could be shorted out in turn until the meter starts to give a very low indication and the reading noted. More resistance could be introduced as the tube warms and livens up. A potential divider across the supply will make the tests easier.There is no fixed rule. Hope this helps. Any old crt could be experimented on including a colour tube providing the heater voltage is known and the connections to each gun. [choice of three!] Its a very gentle procedure and if care is taken can revive a tired tube to almost new brilliance.
Regards John.

Zelandeth · 14th May 2007, 9:36 pm

Cheers for the feedback there, folks.

Isn't the sort of thing I can throw together with random bits and pieces I have laying around then, that high voltage supply (...don't tempt me to string together 10 PP3 batteries!) kinda rules it out for now. An interesting idea though, maybe of use to folks with a bit more in the way of resources handy though.

The lack of a meter able to measure currents that small is a bit of a pain too...though if I know the supply voltage and the resistance in use, can just work it out with Ohm's Law anyway...

For now, I'll just wait until the set's up and running, and I can get at least a couple of hours on it, then see how things go.

Mike Phelan · 15th May 2007, 8:33 am

Anyone that has knocked up a capacitor reformer might find it of use for the G-K reactivating current.

Heatercathodeshort · 15th May 2007, 6:44 pm

Hello again,
A good few companies in the 50's would 'reactivate' your old tube as apposed to rebuilding with a new gun assembly. This would have involved using a similar device to the tube 'tickler' mentioned in the linked thread. The better ones also refired the getter assembly with an RF heater unit. I would imagine that results were variable but cheap.
A complete tube rebuild performs all the operations involved when building a new tube other than the actual building of the gun assembly and the manufacture of the actual bulb. Rebuilders or regunners as they were better known bought ready made gun assemblies.
The screens were rarely renewed and the few firms that did this never reached the standards of the original makers. Often results had a slight greenish tinge.....
The best screens were the Mazda alluminised and the best guns made by Mullard. There were also some in house designs worthy of merit such as the original Ferranti and GEC tubes produced before 1955.
Regards John.

13th May 2007, 12:04 am	#1
Zelandeth Heptode Join Date: Jun 2006 Location: Milton Keynes, Buckinghamshire, UK. Posts: 931	Reviving a "sleeping" CRT We all know the situation. You've got a CRT which has been sitting unused for 20, 30 years...and it's showing up as low emission. We all know that it's worth running the tube for a good length of time (24, 48 hours?) before condeming it though, or risking "boosting" it to see if anything happens. I know that two things happen when it's left running: 1. The emissive coating on the cathode is drawn back to the surface to allow emission to take place properly again. 2. The getter is reactivated by the heat from the heater, which helps to clean up any air (within reason) which may have found its way into the tube in the years since it was last used. ...however, you've got a set which isn't really in a condition to be run for any length of time for risk of self-destructing. Either that, or you don't want something using 200 odd watts of power left running for a day! (Don't know about anyone else...but I know that my old KB does a good impression of a space heater too) This is my question: For this to work, is it just a matter of the CRT heater running, or is the process reliant on the full EHT being present as well? i.e. is it possible to improve things just by connecting the CRT up to a suitable heater supply and leaving it running? Or would one need to actually have it running in the set? I tend to think it's unlikely to be that simple...otherwise everyone with this question would have just found a suitable heater transformer and left it running! Thoughts? __________________ Zel's Lair - A random bag of vintage tech, lighting information and other assorted geekery

13th May 2007, 1:22 am	#2
Zelandeth Heptode Join Date: Jun 2006 Location: Milton Keynes, Buckinghamshire, UK. Posts: 931	Re: Reviving a "sleeping" CRT I know there are ways to revive old, low emission CRTs, at least for a while. What I'm talking about here though is a tube which is quite possibly in good health (but you don't know that for certain), but is really just needing a good old run. Obviously, given that the boosting proceedure has a certain risk attached to it, you'd not want to do that unless you knew the tube was low emission. In which case there's little to lose. In this case, the question is more whether it's actually worthwhile sticking a 6.3V supply onto the heater pins in the CRT (or whatever the correct voltage is for the tube in question), and leaving it to glow away to itself for a day or so. Or without the EHT present, is that pointless? __________________ Zel's Lair - A random bag of vintage tech, lighting information and other assorted geekery

13th May 2007, 3:43 am	#3
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: Reviving a "sleeping" CRT You need to draw some current through the tube as well as the heater . This is done when a tube is new, so some HT is used at a controlled way and the tube used as a diode, but limit the current to microamps. EHT isn't needed to age a tube! Last edited by Mike Phelan; 11th Jun 2007 at 4:58 pm. Reason: Typos

13th May 2007, 8:10 am	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Reviving a "sleeping" CRT Hello, As Danny says, a very small current needs to flow between the cathode and grid to liven up a very tired CRT. This is carried out when the tube is built and is called 'ageing'. As long as there is a current, no matter how small passing between the cathode and grid, the relivening process will slowly take place. It might be worth experimenting with a common tube of any type, a resistor network supplying a very small positive voltage to the grid and a microamp meter connevted in the cathode. [Getters used with vintage tubes were generally large and cope well with any microscopic gas molecules. The later tubes such as the A50-120WR had much smaller assemblies and a visual inspection of the getter area will show a lack of the familiar silvery appearance.] Start with just a few MICROamps and then increase it to say 10 for a few hours. This will not harm the tube in any way. If this fails to produce a result the tube reactivator circuit linked by Steve should do the trick without ruining the tube. Regards, John.

14th May 2007, 7:26 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Reviving a "sleeping" CRT Hello Paul, The voltage needs to be around 30-90v to stimulate the initial 'kick' through a fixed resistor or number of resistors in series. These could be shorted out in turn until the meter starts to give a very low indication and the reading noted. More resistance could be introduced as the tube warms and livens up. A potential divider across the supply will make the tests easier.There is no fixed rule. Hope this helps. Any old crt could be experimented on including a colour tube providing the heater voltage is known and the connections to each gun. [choice of three!] Its a very gentle procedure and if care is taken can revive a tired tube to almost new brilliance. Regards John.

14th May 2007, 9:36 pm	#6
Zelandeth Heptode Join Date: Jun 2006 Location: Milton Keynes, Buckinghamshire, UK. Posts: 931	Re: Reviving a "sleeping" CRT Cheers for the feedback there, folks. Isn't the sort of thing I can throw together with random bits and pieces I have laying around then, that high voltage supply (...don't tempt me to string together 10 PP3 batteries!) kinda rules it out for now. An interesting idea though, maybe of use to folks with a bit more in the way of resources handy though. The lack of a meter able to measure currents that small is a bit of a pain too...though if I know the supply voltage and the resistance in use, can just work it out with Ohm's Law anyway... For now, I'll just wait until the set's up and running, and I can get at least a couple of hours on it, then see how things go. __________________ Zel's Lair - A random bag of vintage tech, lighting information and other assorted geekery

15th May 2007, 8:33 am	#7
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Reviving a "sleeping" CRT Anyone that has knocked up a capacitor reformer might find it of use for the G-K reactivating current. __________________ Mike.

15th May 2007, 6:44 pm	#8
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Reviving a "sleeping" CRT Hello again, A good few companies in the 50's would 'reactivate' your old tube as apposed to rebuilding with a new gun assembly. This would have involved using a similar device to the tube 'tickler' mentioned in the linked thread. The better ones also refired the getter assembly with an RF heater unit. I would imagine that results were variable but cheap. A complete tube rebuild performs all the operations involved when building a new tube other than the actual building of the gun assembly and the manufacture of the actual bulb. Rebuilders or regunners as they were better known bought ready made gun assemblies. The screens were rarely renewed and the few firms that did this never reached the standards of the original makers. Often results had a slight greenish tinge..... The best screens were the Mazda alluminised and the best guns made by Mullard. There were also some in house designs worthy of merit such as the original Ferranti and GEC tubes produced before 1955. Regards John.